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Jeffery Wong %vadmfd1&Rd10 San Joaquin County Environmental Division 22 December 2003 Page 2 Twelve groundwater monitoring wells are currently being sampled as part of an ongoing groundwater monitoring program conducted under the oversight of the SJCED. SITE GROUNDWATER CONDITIONS The release that occurred at the former Tank No 7 location has been identified as being responsible for the groundwater contamination currently present at the site. Three monitoring wells, MW-3, located adjacent to the UST location, and down gradient wells MW-9 and MW-22, have significant concentrations of petroleum hydrocarbons. Up to 23 inches of free product was encountered in a groundwater monitoring well MW-3 in 1992. From 1994 to approximately 1997 there was a regional rise in groundwater of at least 10 feet. From October 1995 to February 1996 the groundwater rose to above the well screen elevation in MW-3, and the free product decreased and finally stopped appearing in MW-3 during this same period. Since groundwater analyses began in MW-3 duringl996, the total petroleum hydrocarbon as gasoline (TPH-g) concentrations have not shown a significant change, and have averaged approximately 180,000 micrograms per kilograms (ug/kg). Attached is page 1 of Table 3, from Advanced GeoEnvironmental's latest quarterly report for the site, which summarizes the analytical results and groundwater elevation data for monitoring well MW-3. PROPOSED ADDITIONAL WORK The foAn-wing data suggg-st%,&t th2tt is thUe is a eantinuing source of gasoline contamination at the former UST No 7 location, and that free product may still be present: 1. The decrease in free product in MW-3 in 1996, corresponded with a rise in groundwater above the well screen elevation. If free product is still present, it is not expected that it would be detected in MW-3, as the well screen currently terminates 5 to 7 feet below the groundwater table. 2. Since groundwater analyses began in MW-3 during1996, the TPH-g concentrations have stayed high, and have not shown a significant change. This is in spite of the fact that, for several years, in-situ remedial technologies (air sparging, vapor extraction and ozone sparging) have been operating at the site. If free product is present in groundwater, or high concentrations of gasoline are still present in soil at the Former UST No 7 location, than continued in-situ remediation may not effectively cleanup the groundwater contamination. Additionally, the current ozone sparging system does not appear to be designed to address high residual contamination around MW-3, as there are not any ozone injection points located within 25 feet of MW-3. If free product or high soil conditions are present, than excavation may be more cost effective than continued in-situ remediation.